The degassing of volatile components from a polymer melt or solution is an important process step in the manufacture and treatment of plastic materials. Of decisive significance for the correct degassing of a specific type of plastic material and for maintaining operational safety is the arrangement and configuration of degassing openings in the housing of the screw extruder. It is known (H. Werner, Degassing of Plastics, Kunststoffe 71 (1981) 1, p. 23, FIG. 11), that degassing housings of various configurations can be utilized with or without ancillary degassing units for the screw extruders, depending on the vapor velocities that are produced and the tendency of the product to foam in the degassing zone in the extruder. The ancillary units can have a configuration which completely or partially covers the "wedge" region of two overlapping extrusion screws. Even if the ancillary degassing units are specially shaped to correspond to the prevailing flows in the degassing zone of the extruder, separation of particles from the plastic melt being processed cannot be avoided, particularly during start-up and when fluctuations in throughput occur. These separated particles adhere to the outer surfaces of the degassing unit and housing as well as on the inner wall of the degassing dome to form solid residues thereat. This produces a constricting of the cross section resulting in higher gas velocities which cause the entrained particles in the gas current to be increased.
The thus formed residues decompose under the effect of temperature and drop, almost decomposed, onto the screws after a certain period of time, and adversely affect the quality of the melt product. In addition, depositions at corners or edges of the degassing unit can become detached and drop into the melt at any time and cause melt contamination.
In order to eliminate the formation of depositions in the degassing dome, it is known from U.S. Pat. No. 3,078,512, to employ a displaceable unit, which has exactly the same contour shape as the degassing dome. The depositions in the degassing dome are removed and reintroduced into the screw extruder by periodic displacement of this unit by a hand lever system.
This system is relatively expensive and even if the degassing dome remains free of depositions by precise monitoring of the formation of depositions and corresponding operation of the displaceable unit, depositions on the surface of the unit facing the screws cannot be eliminated. Unfortunately, this is where most of the plastic particles are deposited.